With increased interest in distributed DC electrical power systems, there is the problem of consolidating and inverting electrical power from diverse DC sources into a single AC power grid. This problem arises upon interconnecting arrays of batteries, fuel cells, solar cells, multi-electrode pair magnetohydrodynamic generators, wind turbine generators or any combination thereof.
Work by R M Johnson and V Skurdall was published in the Proceedings of the 24th Symposium on Engineering Aspects of MHD (SEAM) in Butte, Mont., Jun. 24-27, 1986, entitled PULSE-AMPLITUDE-SYNTHESIS-AND-CONTROL (PASC) OF MHD COMMERCIAL POWER OUTPUT. This work used a digitally controlled thyristor switching matrix connected to switched DC transformers as shown in FIGS 1a and 1b. The same apparatus was also described in (1) 1992 IEEE, 92 SM 520-7 EC, A PULSE-AMPLITUDE-SYNTHESIS-AND-CONTROL (PASC) CONSOLIDATION/INVERSION SYSTEM FOR FARADAY CONNECTED MHD GENERATORS, R M Johnson, M K Donnelly, and K E Marcotte, (2) 1993 IEEE, 93 WM 055-4 EC, A PULSE-AMPLITUDE-SYNTHESIS-AND-CONTROL (PASC) INVERSION SYSTEM FOR SINGLE-SOURCE DIAGONALLY CONNECTED MHD GENERATORS, R Johnson, and M Donnelly, and (3) July 1993, Symposium on the Engineering Aspects of MHD, PULSE AMPLITUDE-SYNTHESIS-AND-CONTROL (PASC) CONVERTER FOR POWER SYSTEM CONNECTION OF A DIAGONALLY CONNECTED MHD CONVERTER, R M Johnson, and M K Donnelly.
Referring now to FIG. 1a, there are two transformers 10, each having two primary windings 12, and each having one secondary winding 14. Each primary winding 12 is connected in parallel to a shorting or snubbing gate 16, in series to a power injection gate 18, and thence in parallel with a DC source 20. Each DC source 20 is independent. The secondary windings 14 are connected in parallel to an AC load 22.
In FIG. 1b, the same components are shown, and in addition, a control computer 24 providing snubbing or shorting signals 26, and injection signals 28 to all shorting or snubbing gates 16 and to all power injection gates 18 according to a control algorithm.
The control algorithm is shown in FIG. 1c. A fixed delay or sequence of delays is used which does not permit synchronous operation. However, a pseudo synchronous operation was achieved by the step of comparing the zero crossing of the stepped wave to the zero crossing of the AC grid. Even with the zero crossing comparison, only 1 or 2 quadrant control was possible. In operation, the control steps 30 provide a control signal (26, 28) to each gate 16 according to the schedule of fixed delays.
Although the PASC inverter is able to consolidate disparate DC power inputs into a single AC output, it is limited to single quadrant or, at best, two quadrant operation and is always lagging and therefore not synchronized to an AC power grid. It will be understood by those of skill in the art of electrical power generation and transmission that "quadrant operation" refers to the quadrants defined by the orthogonal axes of power and reactive power.